Existing colloid thrusters utilize pressure fed capillary emitter geometry to transport liquid to the base of Taylor Cones. FIG. 1 shows a schematic of Taylor Cone formation from a pressure fed capillary emitter. A voltage can be applied to a capillary emitter 10, relative to an electrode 20. The balance between surface tension and electric pressure forms a Taylor Cone 30 and generates emission of ions 40. Droplets can be emitted, due to instability, at apex of cone 50. Droplets can carry most of the ejected mass (i.e., since droplets are relatively heavy) while delivering little impulse (i.e., as droplets move relatively slowly). This can translate into inefficient operation. In ion beam etching, droplets can also contaminate the substrate.
Pressure fed capillary emitters, however, can require pressurization systems (e.g., onboard the spacecraft using the emitters), that adds mass/weight and complexity to the system. The difficulties in fabricating small, uniform capillaries can pose problems in the miniaturization of needle arrays. One way to avoid the issues of pressure fed capillary emitters is to use externally wetted emitter geometries where liquid is drawn from a reservoir by capillary forces. Such passively fed systems can supply liquid at the rate established by the electrospray emission process. The use of externally fed emitters in vacuum, however, is possible with ionic liquids.
Ionic liquids (ILs) are molten salts at room temperature and exhibit extremely low vapor pressures. ILs are formed by positive and negative ions which can be directly extracted and accelerated to produce thrust when used in bipolar operation. ILs have been shown to emit a purely ionic current when exposed to a strong applied potential. ILs generate a substantially pure ionic emission and have a relatively low starting voltage (e.g., less than approximately 2 kV required to generate ions from the Taylor Cone). ILs allow for a scalable specific impulse of the electrospray emitter(s) from approximately 500 seconds to 5000+ seconds. Some ILs can display super-cooling tendencies in which they remain as liquids well below their nominal freezing points. Just as their inorganic cousins (simple salts like NaCl, KBr, etc.) at their melting points (typically >850° C.), ILs exhibit appreciable electrical conductivity at room temperature, making them suitable for electrostatic deformation and subsequent Taylor Cone formation. ILs are thermally stable over a wide range of temperatures (they do not boil, but decompose at temperatures ˜250-500° C.) and are apparently non-toxic being able to be used with applications with green standards, such as in the synthesis and catalysis of chemical reactions. ILs can be used in electrochemical systems, such as in high energy density super-capacitors. ILs' electrochemical window (i.e., the maximum potential difference sustainable by the liquid before electrochemical reactions are triggered) is higher than in conventional aqueous solutions. ILs have low vapor pressures at, or moderately above, their melting points. This allows for use in high vacuum equipment in open architectures such as externally wetted needles/emitters.
Ion sources using ILs can produce positive or negative ion beams with: (1) narrow energy distributions, (2) high brightness, (3) small source size, and (4) wide selection of liquids with diverse molecular compositions. IL ionic sources can be used as a simple and compact source of nearly-monoenergetic negative ions, which can reduce the charge build-up that limits the ability to focus non-neutralized positive ion beams onto dielectrics (insulators or some biological samples) or conductive, but electrically floating targets, and act as a chemically reactive etch agent for materials micro- and nanoprocessing applications.